Compose DOTNET_STARTUP_HOOKS across all Android environment sources instead of overwriting

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

@@ -23,8 +23,7 @@ See: https://github.com/dotnet/sdk/pull/52581
   <!--
     _AndroidConfigureHotReloadEnvironment:
     Configures Hot Reload when dotnet-watch passes environment variables via @(RuntimeEnvironmentVariable).
-    - Adds the Hot Reload agent DLL as a reference so it gets deployed
-    - Updates DOTNET_STARTUP_HOOKS to use just the assembly name (not the full path)
+    - Adds the Hot Reload startup hook assembly name to @(_AndroidDotnetStartupHooks)
     - Sets up STARTUP_HOOKS via RuntimeHostConfigurationOption for MonoVM
   -->
   <Target Name="_AndroidConfigureHotReloadEnvironment"
@@ -40,13 +39,9 @@ See: https://github.com/dotnet/sdk/pull/52581
       <_AndroidHotReloadAgentAssemblyName>$([System.IO.Path]::GetFileNameWithoutExtension('$(_AndroidHotReloadAgentAssemblyPath)'))</_AndroidHotReloadAgentAssemblyName>
     </PropertyGroup>
 
-    <!--
-      Update DOTNET_STARTUP_HOOKS in @(RuntimeEnvironmentVariable) to use just the assembly name.
-      The full path doesn't work on Android since the DLL is deployed alongside the app.
-    -->
+    <!-- The full path doesn't work on Android since the DLL is deployed alongside the app. -->
     <ItemGroup Condition=" '$(_AndroidHotReloadAgentAssemblyName)' != '' ">
-      <RuntimeEnvironmentVariable Remove="DOTNET_STARTUP_HOOKS" />
-      <RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS" Value="$(_AndroidHotReloadAgentAssemblyName)" />
+      <_AndroidDotnetStartupHooks Include="$(_AndroidHotReloadAgentAssemblyName)" />
       <!-- Set STARTUP_HOOKS via RuntimeHostConfigurationOption for MonoVM (read by Mono runtime) -->
       <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="$(_AndroidHotReloadAgentAssemblyName)" Condition=" '$(UseMonoRuntime)' == 'true' " />
     </ItemGroup>

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets

@@ -234,6 +234,7 @@ This file contains the NativeAOT-specific MSBuild logic for .NET for Android.
 
     <GenerateNativeAotEnvironmentAssemblerSources
         Environments="@(AndroidEnvironment);@(LibraryEnvironments)"
+        DotNetStartupHooks="@(_AndroidDotnetStartupHooksDistinct)"
         HttpClientHandlerType="$(AndroidHttpClientHandlerType)"
         OutputSources="@(_PrivateEnvironmentAssemblySource)"
         RID="$(RuntimeIdentifier)" />

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.LlvmIr.targets

@@ -146,6 +146,7 @@
         AdditionalProviderSources="@(_AdditionalProviderSources)"
         AndroidRuntime="$(_AndroidRuntime)"
         Environments="@(_EnvironmentFiles)"
+        DotNetStartupHooks="@(_AndroidDotnetStartupHooksDistinct)"
         HttpClientHandlerType="$(AndroidHttpClientHandlerType)"
         EnableSGenConcurrent="$(AndroidEnableSGenConcurrent)"
         CodeGenerationTarget="$(_AndroidJcwCodegenTarget)"

src/Xamarin.Android.Build.Tasks/Tasks/GenerateAdditionalProviderSources.cs

@@ -31,6 +31,7 @@ public class GenerateAdditionalProviderSources : AndroidTask
 	public string TargetName { get; set; } = "";
 
 	public ITaskItem[]? Environments { get; set; }
+	public ITaskItem[]? DotNetStartupHooks { get; set; }
 
 	// We need to pass these two to the environment builder, otherwise not used
 	// by this task. See also GenerateNativeApplicationSources.cs
@@ -92,6 +93,7 @@ void Generate (NativeCodeGenStateObject codeGenState)
 			// We care only about environment variables here
 			var envBuilder = new EnvironmentBuilder (Log);
 			envBuilder.Read (Environments);
+			envBuilder.AddDotNetStartupHooks (DotNetStartupHooks);
 			GenerateNativeApplicationConfigSources.AddDefaultEnvironmentVariables (envBuilder, HttpClientHandlerType, EnableSGenConcurrent);
 
 			var envVarNames = new StringBuilder ();

src/Xamarin.Android.Build.Tasks/Tasks/GenerateNativeAotEnvironmentAssemblerSources.cs

@@ -14,12 +14,14 @@ public class GenerateNativeAotEnvironmentAssemblerSources : AndroidTask
 	[Required]
 	public string RID { get; set; } = "";
 	public ITaskItem[]? Environments { get; set; }
+	public ITaskItem[]? DotNetStartupHooks { get; set; }
 	public string? HttpClientHandlerType { get; set; }
 
 	public override bool RunTask ()
 	{
 		var envBuilder = new EnvironmentBuilder (Log);
 		envBuilder.Read (Environments);
+		envBuilder.AddDotNetStartupHooks (DotNetStartupHooks);
 
 		// Environment variables are set by Java (code generated in the GenerateAdditionalProviderSources task)
 		// We still want to set system properties, if any

src/Xamarin.Android.Build.Tasks/Tasks/GenerateNativeApplicationConfigSources.cs

@@ -64,6 +64,7 @@ public class GenerateNativeApplicationConfigSources : AndroidTask
 		public string? PackageNamingPolicy { get; set; }
 		public string? Debug { get; set; }
 		public ITaskItem[]? Environments { get; set; }
+		public ITaskItem[]? DotNetStartupHooks { get; set; }
 		public string? AndroidAotMode { get; set; }
 		public bool AndroidAotEnableLazyLoad { get; set; }
 		public bool EnableLLVM { get; set; }
@@ -113,6 +114,7 @@ public override bool RunTask ()
 			// files (generated by us) which weren't present by the time GeneratJavaStubs ran.
 			var envBuilder = new EnvironmentBuilder (Log, EnablePreloadAssembliesDefault, sequencePointsMode);
 			envBuilder.Read (Environments);
+			envBuilder.AddDotNetStartupHooks (DotNetStartupHooks);
 
 			if (_Debug) {
 				envBuilder.AddDefaultDebugBuildLogLevel ();

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/EnvironmentContentTests.cs

@@ -14,6 +14,15 @@ namespace Xamarin.Android.Build.Tests
 	[Parallelizable (ParallelScope.Children)]
 	public class EnvironmentContentTests : BaseTest
 	{
+		class StartupHookEnvironmentProject : XamarinAndroidApplicationProject
+		{
+			protected override string ExtraDirectoryBuildTargetsContent => """
+				<ItemGroup>
+					<RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS" Value="$(MSBuildProjectDirectory)/hotreload/Microsoft.Extensions.DotNetDeltaApplier.dll" />
+				</ItemGroup>
+			""";
+		}
+
 		[Test]
 		[NonParallelizable]
 		public void BuildApplicationWithMonoEnvironment ([Values ("", "Normal", "Offline")] string sequencePointsMode, [Values] AndroidRuntime runtime)
@@ -161,6 +170,41 @@ public void CheckConcurrentGC ()
 			}
 		}
 
+		[Test]
+		public void DotNetStartupHooksAreMergedFromRuntimeEnvironmentVariableAndAndroidEnvironment ()
+		{
+			const string supportedAbis = "armeabi-v7a;x86";
+
+			var proj = new StartupHookEnvironmentProject {
+				IsRelease = false,
+				OtherBuildItems = {
+					new BuildItem.NoActionResource ("hotreload/Microsoft.Extensions.DotNetDeltaApplier.dll") {
+						BinaryContent = () => [],
+					},
+					new AndroidItem.AndroidEnvironment ("startup.env") {
+						TextContent = () => "DOTNET_STARTUP_HOOKS=MyStartupHook.dll",
+					},
+				},
+			};
+			proj.SetRuntime (AndroidRuntime.MonoVM);
+			proj.SetAndroidSupportedAbis (supportedAbis);
+
+			using (var b = CreateApkBuilder ()) {
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+
+				string intermediateOutputDir = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath);
+				List<EnvironmentHelper.EnvironmentFile> envFiles = EnvironmentHelper.GatherEnvironmentFiles (intermediateOutputDir, supportedAbis, true, AndroidRuntime.MonoVM);
+				Dictionary<string, string> envvars = EnvironmentHelper.ReadEnvironmentVariables (envFiles, AndroidRuntime.MonoVM);
+
+				Assert.IsTrue (envvars.TryGetValue ("DOTNET_STARTUP_HOOKS", out string startupHooks), "Environment should contain DOTNET_STARTUP_HOOKS");
+				CollectionAssert.AreEquivalent (
+					new [] { "Microsoft.Extensions.DotNetDeltaApplier", "MyStartupHook.dll" },
+					startupHooks.Split (':'),
+					"DOTNET_STARTUP_HOOKS should merge values from RuntimeEnvironmentVariable and AndroidEnvironment."
+				);
+			}
+		}
+
 		[Test]
 		public void CheckForInvalidHttpClientHandlerType ([Values] AndroidRuntime runtime)
 		{

src/Xamarin.Android.Build.Tasks/Utilities/EnvironmentBuilder.cs

@@ -7,6 +7,8 @@ namespace Xamarin.Android.Tasks;
 
 class EnvironmentBuilder
 {
+	const string DotNetStartupHooks = "DOTNET_STARTUP_HOOKS";
+
 	static readonly string[] defaultLogLevel = {"MONO_LOG_LEVEL", "info"};
 	static readonly string[] defaultMonoDebug = {"MONO_DEBUG", "gen-compact-seq-points"};
 	static readonly string defaultHttpMessageHandler = "System.Net.Http.HttpClientHandler, System.Net.Http";
@@ -48,12 +50,42 @@ public void Read (ITaskItem[]? envItems)
 		}
 	}
 
+	public void AddDotNetStartupHooks (ITaskItem[]? startupHooks)
+	{
+		if (startupHooks == null || startupHooks.Length == 0) {
+			return;
+		}
+
+		var values = new List<string> (startupHooks.Length);
+		foreach (ITaskItem hook in startupHooks) {
+			if (hook.ItemSpec.IsNullOrWhiteSpace ()) {
+				continue;
+			}
+
+			string value = hook.ItemSpec.Trim ();
+			values.Add (value);
+		}
+
+		if (values.Count == 0) {
+			return;
+		}
+
+		AddEnvironmentVariable (DotNetStartupHooks, String.Join (":", values));
+	}
+
 	public void AddEnvironmentVariable (string name, string value)
 	{
+		string escapedName = ValidAssemblerString (name);
+		string escapedValue = ValidAssemblerString (value);
+
 		if (Char.IsUpper(name [0]) || !Char.IsLetter(name [0])) {
-			environmentVariables [ValidAssemblerString (name)] = ValidAssemblerString (value);
+			if (name == DotNetStartupHooks && environmentVariables.TryGetValue (escapedName, out string? existingStartupHooks)) {
+				environmentVariables [escapedName] = MergeDotNetStartupHooks (existingStartupHooks, escapedValue);
+			} else {
+				environmentVariables [escapedName] = escapedValue;
+			}
 		} else {
-			systemProperties [ValidAssemblerString (name)] = ValidAssemblerString (value);
+			systemProperties [escapedName] = escapedValue;
 		}
 	}
 
@@ -108,5 +140,25 @@ public void AddMonoGcParams (bool enableSgenConcurrent)
 		AddEnvironmentVariable ("MONO_GC_PARAMS", enableSgenConcurrent ? "major=marksweep-conc" : "major=marksweep");
 	}
 
+	static string MergeDotNetStartupHooks (string first, string second)
+	{
+		var mergedHooks = new List<string> ();
+		var seenHooks = new HashSet<string> (StringComparer.Ordinal);
+
+		foreach (string hook in first.Split (new char [] { ':' }, StringSplitOptions.RemoveEmptyEntries)) {
+			if (seenHooks.Add (hook)) {
+				mergedHooks.Add (hook);
+			}
+		}
+
+		foreach (string hook in second.Split (new char [] { ':' }, StringSplitOptions.RemoveEmptyEntries)) {
+			if (seenHooks.Add (hook)) {
+				mergedHooks.Add (hook);
+			}
+		}
+
+		return String.Join (":", mergedHooks);
+	}
+
 	static string ValidAssemblerString (string s) => s.Replace ("\\", "\\\\").Replace ("\"", "\\\"");
 }

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1587,11 +1587,31 @@ because xbuild doesn't support framework reference assemblies.
   </PrepareAbiItems>
 </Target>
 
-<Target Name="_GenerateEnvironmentFiles" DependsOnTargets="_AndroidConfigureHotReloadEnvironment">
+<Target Name="_ComposeDotNetStartupHooks"
+		DependsOnTargets="_AndroidConfigureHotReloadEnvironment">
   <ItemGroup>
+    <_AndroidStartupHooksFromRuntimeEnvironmentVariable Include="@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_STARTUP_HOOKS'))" />
+    <_AndroidStartupHooksFromRuntimeEnvironmentVariable Remove="@(_AndroidStartupHooksFromRuntimeEnvironmentVariable)"
+        Condition=" '$(_AndroidHotReloadAgentAssemblyPath)' != '' And '%(_AndroidStartupHooksFromRuntimeEnvironmentVariable.Value)' == '$(_AndroidHotReloadAgentAssemblyPath)' " />
+    <_AndroidDotnetStartupHooks Include="@(_AndroidStartupHooksFromRuntimeEnvironmentVariable->'%(Value)')" />
+  </ItemGroup>
+
+  <RemoveDuplicates Inputs="@(_AndroidDotnetStartupHooks)">
+    <Output TaskParameter="Filtered" ItemName="_AndroidDotnetStartupHooksDistinct" />
+  </RemoveDuplicates>
+
+</Target>
+
+<Target Name="_GenerateEnvironmentFiles" DependsOnTargets="_ComposeDotNetStartupHooks">
+  <PropertyGroup>
+    <_AndroidDotnetStartupHooksValue>@(_AndroidDotnetStartupHooksDistinct,':')</_AndroidDotnetStartupHooksValue>
+  </PropertyGroup>
+  <ItemGroup>
+    <RuntimeEnvironmentVariable Remove="DOTNET_STARTUP_HOOKS" />
     <_GeneratedAndroidEnvironment Include="mono.enable_assembly_preload=0" Condition=" '$(AndroidEnablePreloadAssemblies)' != 'True' " />
     <_GeneratedAndroidEnvironment Include="DOTNET_MODIFIABLE_ASSEMBLIES=Debug" Condition=" '$(AndroidIncludeDebugSymbols)' == 'true' and '$(AndroidUseInterpreter)' == 'true' " />
     <_GeneratedAndroidEnvironment Include="DOTNET_DiagnosticPorts=$(DiagnosticConfiguration)" Condition=" '$(DiagnosticConfiguration)' != '' " />
+    <_GeneratedAndroidEnvironment Include="DOTNET_STARTUP_HOOKS=$(_AndroidDotnetStartupHooksValue)" Condition=" '$(_AndroidDotnetStartupHooksValue)' != '' " />
     <!-- RuntimeEnvironmentVariable items come from 'dotnet run -e NAME=VALUE' -->
     <_GeneratedAndroidEnvironment Include="@(RuntimeEnvironmentVariable->'%(Identity)=%(Value)')" />
   </ItemGroup>
@@ -1734,6 +1754,7 @@ because xbuild doesn't support framework reference assemblies.
       MonoComponents="@(_MonoComponent)"
       EnvironmentOutputDirectory="$(IntermediateOutputPath)android"
       Environments="@(_EnvironmentFiles)"
+      DotNetStartupHooks="@(_AndroidDotnetStartupHooksDistinct)"
       AndroidAotMode="$(AndroidAotMode)"
       AndroidAotEnableLazyLoad="$(AndroidAotEnableLazyLoad)"
       EnableLLVM="$(EnableLLVM)"